This invention relates to an apparatus for the preparation of photographic silver halide emulsions by a continuous process, in which a stream of liquid entering a conduit system flows successively through the various sections of the system corresponding to the individual stages of the process, such as the inlet point for the metered streams of liquid, the mixing paths and the ripening paths.
The continuous process can easily be developed from the known batchwise process. Instead of the individual stages of the process taking place one after the other in the same place as in the batchwise process, in this case they take place one at successive locations in a conduit system.
The stream of halide solution containing gelatin which is introduced at the inlet of the conduit system corresponds to the so-called reaction medium in the batchwise process. The inlet points for the addition of silver nitrate solution or other additives and the associated mixing paths and ripening paths are arranged to correspond to the time sequence of the various stages of the batchwise process taking place in a reaction vessel.
The number of streams of liquid added and their proportion to each other must correspond to the formulation for the preparation of the particular emulsion.
The length of the ripening paths must correspond to the ripening time of the given recipe. If there is any change in the recipe, the number of inlet points, the metering connections per inlet point, the quantities added and the length of the ripening paths must be adapted to the new recipe.
The continuous preparation of light-sensitive photographic silver halide gelatin emulsions is already known. In German Pat. No. 1 000 686 there is described a continuous preparation of light-sensitive photographic silver halide emulsions ready for casting by introduction of solutions of the individual components into a closed, lightfast tubular apparatus which is subdivided into several production zones, in which the individual stages of the process, such as precipitation of silver halide, physical ripening, washing, chemical after-ripening and finishing to make the emulsion ready for casting on the layer substrate, take place.
In the aforesaid process, gelatin solutions containing potassium bromide are continuously mixed with silver nitrate solution in a filling zone which is equipped with a stirrer apparatus. The following ripening zone is equipped with stirrer screws. The uniformity of dwell time of all the particles of emulsion in the ripening zone which should be achieved in a satisfactory batchwise process after vigorous mixing of the components is only incompletely attained. Moreover the installation of conveyor screws in each section of the ripening path would appear to be very complicated and expensive and unsuitable for rapid cleaning of the apparatus when changing the reaction mixture.
In German Pat. No. 1 106 168 there is described an apparatus for the continuous preparation of photographic emulsions in a tank process by means of a plurality of small batches, in which the tanks are successively moved at regular time intervals to the individual operating stations in order to carry out the essential processes for preparation of the emulsion. This is a continuous process only in its practical outcome but as regards the technique of the process it is a discontinuous, batchwise process with all the known disadvantages which this entails.
An improvement by using larger batches is disclosed in German Offenlegungsschrift No. 1 472 745 which describes a process for the preparation of light-sensitive silver salts which are sparingly soluble in water by the precipitation of water-soluble silver salts with water-soluble metal salts, in which process the dispersion is prepared in two stages.
Precipitation is carried out in a relatively small precipitation space, for example in a pump, by very vigorous mixing of the aqueous solutions of the precipitation components, and the resulting dispersion is then immediately transferred to a ripening chamber of considerably larger volume in which physical ripening is carried out.
Although this method of preparation is a considerable improvement compared with the usual batchwise tank process in providing larger quantities of uniform product, it still has the disadvantage of uneven mixing in the large tank, and the small units of emulsion are not all identical to each other, especially since the dwell times in the tank may vary considerably. Moreover, continuous production is not possible.
The U.S. Pat. No. 3 655 166 describes a continuous process for the preparation of emulsions in which the basic component is carried in an ascending stream inside a closed tubular apparatus and the other components are added successively at cyclically symmetrical points in a transverse direction of flow to the reaction mixture which is in the process of being formed from the main component.
This process and apparatus provide important improvements with regard to the uniformity and continuity of production as well as the possibility of a large number of variations in the addition of components and the ripening time of the particular reaction mixture.
The process has, however, the disadvantage that the mixing space and the ripening space are not clearly separated from each other so that the flow in the ripening space is disturbed by the mixing flow and the particles of emulsion entering at any given point therefore do not have a uniform through-flow time or ripening time. Moreover, adjustment of the apparatus to another recipe with a different ripening time by dismantling and reassembly of part of the pipe elements is too complicated, particularly if the apparatus is very high because it is designed for long ripening times.
The U.S. Pat. No. 3 728 280 is a futher development of the U.S. Pat. No. 3 655 166. It describes an improved method of mixing, in which vibrations in the axial direction of the tubular apparatus are superimposed on the radial transverse stream. In addition, the height of the apparatus is reduced by curving the pipes which are made partly of rigid, partly of elastic sections. However, the disadvantage of interference of the ripening stream by the mixing stream and hence lack of uniformity of the ripening time of the particles of emulsion remains.
The U.S. Pat. No. 3 779 518 describes a process for the continuous preparation of photographic emulsions in which the individual components are introduced into a tubular apparatus together with the crude emulsion and distributed within each other, the individual components being continuously added successively to the main stream, and whichever component has just been added is completely mixed with the main stream in the mixing zone immediately after its entry into the main stream. This mixing may take place in a static mixing path with secondary swirling flow or according to U.S. Pat. No. 3 827 888, FIGS. 2 and 3.
However, this process does not constitute complete preparation of an emulsion since it is only used for making a crude emulsion ready for casting and therefore has no provisions for ripening times or ripening paths.
In the preparation of emulsions, the required spectrum of particle sizes in the finished emulsion plays an important role. In many cases, a very narrow spectrum of particle sizes, i.e. uniformity of particle size is required. This means that all the particles of liquid must undergo the same changes in state with time after entering the process. The continuous flow process appears to be particularly suitable for achieving this if the following conditions are observed:
(a) Each mixing process must be carried out very intensively in view of the chemical reaction associated with it, and should be completed within the shortest possible time. This means that the volume of mixing chamber should be small.
(b) In the ripening paths, there must be a close approximation to a rectangular velocity profile because only in this way is it possible to ensure that all the liquid particles will have the same through-flow time. A velocity profile in the form of a Poiseuille's parabola would be unsuitable. The rectangular velocity profile must on no account be disturbed by remote action from the mixing paths, still less by the stirrer in the ripening path.
Requirements (a) and (b) indicate that the mixing and ripening zone must be clearly separated from each other.
Although homogeneous emulsions with a narrow particle size spectrum are sometimes required, in other cases emulsions are required to have a wide particle size spectrum, i.e. differing particle sizes. In practice, this means differing dwell times of the particles in the ripening paths. However, it must be possible to control these dwell times.
Other important practical requirements include simple and rapid adjustment of the apparatus, i.e. particularly adjustment of the length of the ripening paths to the recipe and ease of cleaning.